Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within cells. Almost all kinases contain a similar 250 to 300 amino acid catalytic domain. The kinases can be categorized into families by the substrates they phosphorylate.
JAK2 (Janus kinase 2) is a family of intracellular non-receptor tyrosine kinases. JAK2 is expressed ubiquitously in hematopoietic cells and abundantly in primary leukemic cells from children with acute lymphoblastic leukemia. The downstream substrates of JAK include the signal transducer activator of transcription (STAT) proteins. STAT proteins function both as signaling molecules and transcription factors and ultimately bind to specific DNA sequences present in the promoters of cytokine-responsive genes. JAK/STAT signaling has been implicated in the mediation of many abnormal immune responses such as allergies, asthma, autoimmune diseases such as transplant (allograft) rejection, rheumatoid arthritis, amyotrophic lateral sclerosis and multiple sclerosis, as well as in solid and hematologic malignancies such as leukemia and lymphomas.
Spleen tyrosine kinase (syk) is a member of the syk family of protein tyrosine kinases and plays a crucial role in inflammatory and allergic responses. Syk triggers IgE and IgG receptor mediated signaling in mast cells, basophils, and macrophages leading to degranulation and cytokine release.
Immunoreceptor tyrosine activation motif (ITAM)-mediated signaling has emerged as a primary event in signaling pathways responsible for human pathologies. ITAM-mediated signaling is responsible for relaying activation signals initiated at classical immune receptors such as T-cell receptors, B-cell receptors, and Fc receptors in immune cells and at GPVI and FcγRIIa in platelets to downstream intracellular molecules such as Syk.
The binding of a ligand to an ITAM-containing receptor triggers signaling events which allows for the recruitment of proteins from a family of nonreceptor tyrosine kinases called the Src family. These kinases phosphorylate tyrosine residues within the ITAM sequence, a region with which the tandem SH2 domains on either Syk or ZAP-70 interact. The interaction of Syk with diphosphorylated ITAM sequences induces a conformation change in the kinases that allows for tyrosine phosphorylation of the kinase itself.
Not only do these kinases contribute to normal host defense, they also play roles in the pathogenesis of diseases. Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events. These diseases include autoimmune diseases, inflammatory diseases, bone diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies, asthma, Alzheimer's disease and hormone-related diseases. As a consequence, there have been substantial efforts in medicinal chemistry to find inhibitors of protein kinases for use as therapeutic agents.
There is a need in the art for compounds that are dual inhibitors of Syk/JAK2, as well as for methods for treating conditions that can benefit from such inhibition.